An echelle spectrograph is an optical instrument that uses an echelle grating to diffract light with high dispersion and utilizes higher diffraction orders. As with other blazed diffraction gratings, the echelle grating contains a number of grooves. However, echelle gratings are specifically characterized by the large spacing between the grooves and, therefore, are characterized with a lower groove density than standard blazed gratings that are designed to be used in the 1st diffraction order.
Light incident upon any ruled grating is split into several different diffraction orders. Each order is comprised of a different wavelength range that overlaps onto the same spatial location as light that is diffracted into other orders. The dispersion associated with each order is also different. The overlapping ranges of orders diffracted from the grating make it difficult to associate a particular wavelength with a given spatial location in the diffracted light. This ambiguity complicates the output spectrum and makes it more difficult to determine the correct wavelength emission produced by the source.
Although this overlap between diffraction orders is generally an unwanted side effect, echelle gratings specifically use this effect to enhance the performance of the spectrograph. To this end, a second cross-dispersing element is used to spatially separate the orders. The individual diffraction orders, each with a separate (and sometimes overlapping) wavelength range and resolution, can then be analyzed without ambiguity.
A lens-based spectrograph can have good resolution and very high throughput (˜f/2) over a limited wavelength range. If the wavelength range of operation needs to be shifted from the design wavelength of the lenses, or if a broad wavelength range is required to be simultaneously acquired such as with an echelle spectrograph, then chromatic aberration limits spectral resolution of a lens-based instrument.
Typical broadband, all-reflective echelle spectrographs have a relatively high f/number, generally f/7 or greater camera focusing optics, limiting the total light that reaches the image plane and thereby decreasing the resulting image quality. Further, the high f/number of a typical echelle grating-based spectrograph prevents the use of such an instrument in certain applications such as Raman spectroscopy, where the detection of weak levels of light emission necessitates the use of a spectrograph with a very low f/number.
A linear array spectrograph uses a standard ruled grating, usually (but not always) in the 1st order. A 1-D linear array sensor is combined with the spectrograph to make a very compact and inexpensive spectrometer. These instruments have limited wavelength coverage but can be appropriate for some applications such as Raman spectroscopy where a limited wavelength range is possible. All-reflective, linear array spectrographs usually implement camera focusing optics that are f/4 or slower, plus the linear array length and resolution can be limited by the quality of the camera focusing optics.
An imaging spectrograph is similar to a linear array spectrograph except that it utilizes a 2-D sensor. A tall entrance aperture can be used with an imaging spectrograph because the image plane is better corrected than a linear array spectrograph in the direction perpendicular to the grating dispersion. The tall entrance aperture permits either much better throughput or multiple fiber inputs aligned along the entrance aperture. The multiple fiber inputs can direct light from various light sources enabling simultaneous monitoring of multiple input channels. The tall slit allows the spectrograph to monitor wavelength information along one axis, while simultaneously measuring spatial information along the other axis. All-reflective imaging spectrographs are typically f/4 or slower, plus the size of the 2-D image plane is limited.